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should project a little beyond the open end of the metal cylinder
in which the sealed tube is enclosed. The temperature, indicated,
by a thermometer fixed in the top of the furnace, is carefully
regulated. It is advisable to commence the operation in the
morning. The temperature is gradually raised from 150° to 200° -
during four hours, and then to 230° for a further four hours. The
gas is then extinguished, and the tube allowed to cool until the
following morning.
Opening the Sealed Tube.—The tube is drawn a little
way out of the iron casing, so that the capillary end projects
3 or 4 cm. The tip is then warmed cautiously in the Bun sen
flame to expel the liquid which as a rule condenses there. The
point is then heated until the glass softens, when the pressure
inside perforates the glass and nitrous fumes are evolved. O?i no
account must the tube be removed from the furnace before //its
operation is concluded. The tube is now taken away and
opened. A deep file scratch is made in the wide part of the
tube, about 3 cm. below the capillary. The end of a glass
rod, heated to redness, is then held against the file mark. A
crack is produced, which may be prolonged round the tube
by touching the tube in front of the crack with the hot end
of the glass rod. The top of the tube is now easily removed ; •
but in order to prevent fragments of glass from the broken
edge from dropping into the acid, the tube should be lielcl
horizontally and the end carefully broken off. Any bits of
glass which become detached adhere to the side of the tube,
near the open end, and can be easily wiped off. The contents
of the tube containing the silver halide are now carefully
diluted by adding water a few c.c. at a time, and then washed
into a beaker. The mixture is heated to boiling, the silver
compound transferred to a filter, and washed with hot water
until free from silver nitrate. The filter paper is then dried
in a steam oven and the silver salt weighed. A simpler and
more accurate method for filtering and weighing the silver
halide is to use a perforated or Gooch crucible. A disc of
filter paper is cut with a cork cutter of suitable dimensions
to fit the bottom of the crucible, which is dried with the crucible
in a Victor Meyer air-bath (Fig. 26) heated to 140—150° until
constant. The air-bath consists of a jacketed copper vessel
fixed upon a tripod. A liquid of constant boiling-point is poured